Hm. Possible. I can see two scenarios. Factory ran out of old 699 screw down cases with countdown bezel and issued some 1624 movements with old dials in newer 10ATM snap back case (they are completely compatible). Or later marriage of older movement and newer case. Obviously we need more statistic.

Actually, just realized I confused JMH's photo with Yuri's. The '55 catalog shows JMH's in both countdown and elapsed time but not like Yuri's with what has become the classic Sea Wolf bezel layout (15 min timer and triangles at 3min intervals. However, still would like to know when the case change over occurred. Did it correspond with the elevated depth rating.